In recent years, worldwide restrictions on exhaust gas are becoming tighter from the viewpoint of environmental protection. As one measure, exhaust gas purification catalysts are being employed in internal combustion engines. In order to efficiently remove the hydrocarbons (hereunder abbreviated as “HC”), CO and nitrogen oxides (hereunder abbreviated as “NOx”) in exhaust gas, exhaust gas purification catalysts employ precious metals such as Pt, Pd and Rh as catalyst components.
Vehicles using such exhaust gas purification catalysts, such as gasoline engine vehicles and diesel engine vehicles, employ various types of systems designed to increase both catalytic activity and fuel efficiency. For example, in order to increase fuel efficiency, combustion is carried out under lean air/fuel ratio (A/F) conditions (oxygen excess) during steady operation, and in order to increase catalytic activity, combustion is temporarily conducted under stoichiometric (theoretical air/fuel ratio, A/F=14.7) to rich (fuel excess) conditions.
This is because conventionally known catalysts including precious metals such as Pt, Pd and Rh have low NOx purification performance under oxidizing conditions, and require a reducing atmosphere by addition of HC or CO to increase purification performance. There is consequently a limit to the increased fuel efficiency that can be achieved with catalysts of precious metals and the like.
With conventionally known catalysts of precious metals and the like, it has been necessary to use fuels that temporarily bring the purification catalyst into a reducing atmosphere. Also, in order to increase the fuel efficiency of internal combustion engines such as automobile engines, it has been a goal to obtain novel purification catalysts that can exhibit NOx purification performance under lean atmosphere conditions, for example.
There have also been attempts at modifications to increase the performance of NOx purification catalysts.
PTL 1 (Reference 1, claim 1 and elsewhere) describes an exhaust gas purification method wherein nitrogen oxide-containing exhaust gas, together with ammonia, is passed over a catalyst comprising 60 to 99.9 wt % of titanium-containing oxides, 0.1 to 20 wt % of at least one element from among copper, manganese and chromium and 0 to 20 wt % of at least one element from among vanadium, tungsten, molybdenum and tin, at a temperature of 200° C. to 500° C.